The fire book () describes battle itself. The similarities to the Art of War are striking. This shouldn't surprise. Strategy is universal or there wouldn't be any point in us reading these old books now. There's no way to know if Musashi copied the ideas, discovered them independently, or was paraphrasing. Musashi advocates driving the enemy towards difficult terrain and to "win through the use of the place itself".
He explains something called "Holding down the pillow" that you can in any Kendo Dojo. Kendo is Japanese fencing. Sometimes you get a chance to fight Japanese players well over 50 years old fight the 20 some things. In spite of their aged physique, they always win. At the end of the 3 minute round the young player takes their mask off and you can see their red, sweaty face panting. The older player will have little more than a mild sweat. This is because they understand "Holding Down the Pillow".
The point is to allow your opponents' useless actions. These actions use their resources and achieve nothing. Instead, cut off their useful actions before they are even executed. This requires speed and perception, but it is easier if you allow their useless actions. By the way, a Japanese pillow looks like this porcelain example on the right.
This chapter also has the popular river crossing lesson. The idea of this lesson is that sometimes in life we make a plan to do something significant. This is likened to crossing a river at a ford. Although the thing is difficult to do, we have planed and the time has arrived, so we need the courage of our convictions. We need to cross.
If things go badly, and we are near the goal, we need to try harder. This is compared to crossing in a sail boat when the wind stops. We must simply start rowing. Musashi's metaphor brings to mind another old Japanese saying: "When the tide rises so does the boat." That is, when circumstances require it, we must increase our efforts.